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A Tour of NTL: Tips for Getting the Best Performance out of NTL 
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A Tour of NTL: Tips for Getting the Best Performance out of NTL
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<p> <hr> <p>

<ol>

<li>
Build NTL using GMP as the primary long integer package.
This is extremely important, as the GMP implementation
of long integer arithmetic is <i>much</i> faster
than the default implementation.
Go <a href="tour-gmp.html">here</a> for details.

<p>
<li>
On many machines that optionally offer 64-bit integer arithmetic
(recent Mac OSX machines, for instance),
you should
compile using <tt>gcc</tt> with the option <tt>-m64</tt>
to get the full benefit.
To do this,
pass <tt>"CFLAGS=-O2 -m64"</tt>
to the <tt>configure</tt> script (note the use of quotes).
If you are using NTL with GMP on such a machine,
you <i>must</i> do this to get compatible code.

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<li>
On Sparcs, 
pass the argument <tt>"CFLAGS=-O2 -mcpu=v8"</tt>
to the <tt>configure</tt> script.
On more recent, 64-bit sparcs, pass <tt>"CFLAGS=-O2 -mcpu=v9 -m64"</tt>
to get the full instruction set and 64-bit code.

<p>
<li>
Make sure you run the configuration wizard when you install NTL.
This is the default behaviour in the makefile
in the Unix distribution, so don't change this;
in the Windows distribution, there is unfortunately no 
easy way to run the wizard.

<p>
<li>
In time-critical code, avoid creating unnecessary temporary
objects.
For example, instead of
<pre>
ZZ InnerProduct(const ZZ *a, const ZZ *b, long n)
{
   long i;
   ZZ res;
   for (i = 0; i < n; i++)
      res += a[i] * b[i];
   return res;
}
</pre>
write this as
<pre>
ZZ InnerProduct(const ZZ *a, const ZZ *b, long n)
{
   long i;
   ZZ res, t;
   for (i = 0; i < n; i++) {
      mul(t, a[i], b[i]);
      add(res, res, t);
   }
   return res;
}
</pre>
The first version of <tt>InnerProduct</tt>
creates and destroys a temporary object, holding the value
<tt>a[i]*b[i]</tt>, in every loop iteration.
The second does not.




<p>
<li>
If you use the class <tt>ZZ_p</tt>, try to avoid switching the modulus
too often, as this can be a rather expensive operation.
If you <i>must</i> switch the modulus often,
use the class <tt>ZZ_pContext</tt> to save the information
associated with the modulus (see <a href="ZZ.txt">ZZ.txt</a>). 



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